Backwash method and apparatus

ABSTRACT

A method and apparatus for cleaning filter elements that are vertically positioned within a filter vessel that has a tube sheet dividing the filter vessel into a lower filter compartment and an upper plenum compartment. A fixed volume of backwash liquid is rapidly directed from the plenum compartment above the tube sheet through the filter elements into the filter compartment that has been drained of liquid. The rapid movement of the backwash liquid is a result of a pressure differential between the plenum compartment and the filter compartment. The volume of backwash liquid is maintained above the tube sheet by a pressurized condition in the filter compartment, which volume may be changed by adjusting the pressure in the filter compartment.

FIELD OF THE INVENTION

This invention relates generally to a method and apparatus for cleaninga plurality of filter elements that are vertically positioned within afilter vessel. More specifically, the invention relates to a method andapparatus for cleaning such filter elements positioned within a filtervessel that has a tube sheet dividing the filter vessel into a lowerfilter compartment and an upper plenum compartment.

DESCRIPTION OF THE RELATED ART

The method and apparatus in accordance with the invention is disclosedin conjunction with a specific filter apparatus that utilizes precoatson filter elements. It is contemplated that the invention may besimilarly utilized in conjunction with other types of filter apparatusthat do not utilize precoats on filter elements.

It is known that liquids may be very effectively purified by passingthem through a filter screen which has been precoated with a layer ofparticles, referred to herein as "precoat layer", such as ion exchangeresin particles. Such precoat layer may remove dissolved solids andundissolved solids when the precoat layer contains active materials suchas ion exchange particles. This method is disclosed in U.S. Pat. No.3,250,703, issued May 10, 1966, and assigned to the same assignee ofthis application. Periodically, it becomes necessary to clean the filterbecause the precoat layer's adsorbent or absorbent capacity has beenexhausted or the filter's resistance to flow has increased due to anaccumulation of particles in or on the precoat layer, or within thefilter elements, or both.

The filters normally employed are filter elements that have cylindricalcores with apertures to allow flow to pass through the walls of thecylinder. The core may be made of perforated metal, molded plastic,sintered powdered metal or plastic, laminated layers of woven wire metalor plastic, or other constructions providing adequate tensile andcompressive strength and permeability. The filter elements are typicallypositioned within a filter vessel and are secured to a tube sheet thatdivides the filter vessel into a filter compartment and a plenumcompartment. There are generally two types of such filter apparatus thatemploy vertically positioned elements; one has the plenum compartmentabove the filter compartment (top tube sheet filter), an example of suchtype is disclosed in U.S. Pat. No. 3,637,079. The other type has theplenum compartment below the filter compartment (bottom tube sheetfilter), an example of such type is disclosed in U.S. Pat. No.3,666,097. The present invention is particularly directed to cleaningthe filter elements in the former type of filter apparatus.

It is the well known practice to clean the filter elements in a bottomtube sheet filter vessel by use of a series of steps that include aseries of backwash cycles to force a high velocity flow of cleaningliquid through the filter elements in the reverse direction of theservice flow at high velocities. The high velocity may be imparted tothe cleaning liquid by use of gas from a pressurized source. Such abackwash cycle is commonly known as a gas or air surge backwash cycle.The volume of cleaning liquid that is forced through the filter elementsduring each backwash cycle is approximately equal to the volume ofcleaning liquid that is in the plenum compartment above the lower endsof the tube sheet fittings associated with the filter elements. Inexisting filter apparatus where the volume of the plenum area is fixed,and in newly designed filter apparatus where limitations are placed onthe vessel length or tube sheet fitting length, the volume of cleaningliquid flow during each cycle is accordingly limited.

In U.S. Pat. No. 4,405,466, assigned to the same assignee as the presentapplication, a backwash method and apparatus is disclosed for a bottomtube sheet type of filter. The backwash system directs an externalvolume of backwash liquid into the plenum compartment and through thefilter elements under gas. pressure. The volume of cleaning liquiddirected through the filter elements during each backwash cycle is inthe range of 1.0 to 3.0 times the total element core void volume of thefilter elements.

The heretofore proposed backwash systems for top tube sheet filters havebeen limited to backwash flow rates that have proven to be inadequatefor small retention rated filter elements. Further, in many applicationscapacities of waste treatment systems required to process liquidsdischarged during backwashing is limited, making numerous repeats of thebackwash cycle impractical.

Such backwash systems typically start with a liquid filled vessel,except for a pressurized gas dome in a portion of the upper plenumcompartment. The energy available for backwashing is limited to theenergy stored in the pressurized gas. This energy is expended inovercoming: (1) the inertia of the entire mass of liquid in the vesseland (2) the entire system's resistance to flow. It is inherent in suchbackwash systems that the majority of the limited energy is wasted inthe movement of the liquid in the lower filter compartment whichprovides no backwash benefits. The magnitude of this wasted energy maybe gauged by considering that during a backwash, the pressure dropthrough the bottom drain valve and piping may be 10 to 15 times thatthrough the filter elements, and the mass of liquid in the lower filtercompartment may be about 3 times that in the upper plenum compartment atthe start of a backwash cycle.

Accordingly, there is a need for an improved backwash method andapparatus for cleaning the filter elements in a top tube sheet filterthat more effectively directs the energy to the backwash liquid tocreate a driving force for the movement of the backwash liquid in arapidly moving piston-like flow of liquid surge downwardly through thecore of the filter elements and radially outward through the filtersurfaces thereof.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided forcleaning a plurality of filter elements, which are vertically positionedwithin a filter vessel having a tube sheet dividing the filter vesselinto an upper plenum compartment and a lower filter compartment. Thefilter elements extend downwardly into the filter compartment and haveupper end portions that are in fluid communication with the plenumcompartment. Briefly stated, the method includes the following steps:draining at least a portion of the liquid from the filter vessel;establishing a pressurized condition in the filter compartment and thefilter elements that is sufficient to support a predetermined volume ofbackwash liquid in the plenum compartment above the tube sheet; andincreasing the pressure in the plenum compartment while reducing thepressure in the filter compartment so as to cause the fixed volume ofbackwash liquid to move rapidly downward through the cores of the filterelements and surge radially outward through the filter surfaces thereof.The preceding steps of the method may be repeated a plurality of times.

The apparatus of a preferred form of the invention includes a filtervessel having a tube sheet dividing the vessel into an upper plenumcompartment and a lower filter compartment. A plurality of verticalprecoat filter elements extend downward from the tube sheet into thefilter compartment and have upper portions that communicate with theplenum compartment through openings formed in the tube sheet. The filtervessel includes a drain for draining liquid and slurries from the bottomof the vessel. A source of pressurized gas is in communication with thefilter compartment for selectively establishing a pressured condition inthe filter compartment and the filter elements that is sufficient tosupport a predetermined volume of backwash liquid in the plenumcompartment above the tube sheet. A backwash liquid supply linecommunicates with the plenum chamber through which a predeterminedvolume of backwash liquid is directed into the plenum compartment. Asource of pressurized gas communicates with the plenum compartment toincrease the pressure therein. A vent means is provided to reduce thepressure in the filter compartment so as to cause the fixed volume ofbackwash liquid to move rapidly downward through the cores of the filterelements and surge radially outward through the filter surfaces thereof.

In a preferred form of the apparatus, the upper portions of the filterelements extend a short distance into the plenum compartment and a capmember is supported in covering relationship and spaced from the upperportions of the filter elements. The cap members have lower end portionsspaced a short distance above the tube sheet. The source of pressurizedgas that communicates with the filter compartment is preferablycontrolled by a regulator means to selectively control the gas pressurein the filter compartment.

DESCRIPTION OF THE DRAWING

The invention, both as to its organization and method of operation,taken together with the objects and advantages thereof, will be bestunderstood by reference to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a prior art filter apparatusdesigned to employ a prior art backwash method; and

FIG. 2 is a schematic representation of a filter apparatus embodying thefeatures of the present invention and designed to employ the backwashmethod of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method and apparatus of the present invention are described below inconjunction with the filter apparatus schematically illustrated in FIGS.2 and 3. It being understood that the present invention contemplates theuse of alternative apparatus in practicing the teachings of theinvention.

Referring to FIG. 1, a prior art filter apparatus 10 is schematicallyshown. Filter apparatus 10 includes a filter vessel 12 having an inletfeed line 14 and an outlet filtrate line 16. The vessel 12 is dividedinto an upper plenum compartment 18 and a lower filter compartment 20 bya tube sheet 22, suitably secured to the interior of the vessel 12 bywelding or the like. Mounted vertically within the filter compartment 20are a plurality of precoat filter elements 24 through which an influentstream must pass before entering the plenum compartment 18 and beingdischarged from the vessel through outlet line 16. The vessel isprovided with a vent line 26 in communication with an upper portion ofthe plenum compartment 18 and a drain level line 28 in communicationwith an intermediate portion of plenum compartment 18. Lines 14, 16, 26and 28 are provided with suitable valves.

When the precoat on the filter elements 24 becomes exhausted, theservice cycle is stopped and a backwash cycle is initiated. At the startof the backwash cycle, the liquid level in the vessel is drained down tothe level of line 29 so as to create a fixed volume of backwash liquidabove tube sheet 22 in plenum compartment 18. The upper portion ofcompartment 18 is pressurized by directing a gas thereinto through apressurized gas supply line 30. Opening of the bottom drain line 32causes the backwash liquid in plenum compartment 18 to move downwardlythrough the cores of the filter elements and radially outward into thefilter compartment. The backwash liquid, the liquid in the filtercompartment, and the expelled filter cake and precoat layer are directedfrom the vessel through line 32. Much of the energy available from thepressurized gas to enhance the velocity of the backwash liquid isexpended in overcoming the inertia of the liquid in the filtercompartment and the system's resistance to flow.

Referring to FIG. 2, the filter apparatus in accordance with a preferredembodiment of the invention is indicated generally by the referencenumeral 50. The apparatus 50 includes a filter vessel 52 having an inletor feed line 54, controlled by a feed valve 55, and an outlet orfiltrate line 56, controlled by an outlet valve 57. The filter vessel isa generally cylindrical vessel made of steel or the like, and having atop 58 suitably shaped for the design pressure and a convex or coneshaped bottom 60. The vessel is divided into a lower, influent or filtercompartment 62 and an upper, filtrate or plenum compartment 64 by a tubesheet plate 66, suitably secured to the interior of the vessel 52.

Mounted vertically within the filter compartment 62 are a plurality offilter cartridges or elements 68 (only one of which being shown),through which the influent stream must pass before entering the plenumcompartment 64 and being discharged from the vessel through the outletline 56. The filter elements 68 are typically cylindrical in shape withclosed ends 70. The filter elements 68 have suitable mounting means (notshown) for securing the elements to the tube sheet 66 at a correspondingopening 72 formed therein.

A plurality of filter elements 68 are suitably spaced within vessel 52and suitably spaced from the sidewalls of the vessel. The top 58 isprovided with a vent line 74 that is controlled by a vent valve 75. Thebottom 60 is provided with a drain line 76 that is controlled by a drainvalve 77.

The filter apparatus 50 of the present invention will normally be usedto treat a stream of liquid, such as condensate water in the steamgenerating system of an electrical power plant. As those skilled in theart will understand, however, the filter apparatus 50 may be used totreat other liquids.

In preparing the filter apparatus for operation, the initial step is toprecoat the filter elements 68. In order to accomplish this, the filtervessel 52 is filled with a low-impurity water, such as demineralizedwater. A slurry of precoat medium and demineralized water is prepared ina precoat system (not shown). The precoat slurry is directed through theinlet line 54 into the filter compartment 62 of the filter vessel 52.The pressure of the incoming slurry forces the demineralized water inthe filter compartment 62 through the filter elements 68 and out thefilter vessel 52 via the plenum compartment 64 and the outlet line 56and back to the precoat system. The precoat material is retained on theouter surface of the filter element 68. After virtually all of theprecoat material is deposited on the elements 68, the precoating cycleis complete. The thickness of the precoat layer varies, but it ispreferably in the range of about one-quarter inch to no more than fiftypercent of the distance between adjacent elements. The filter apparatus50 is now ready to treat contaminated liquids.

In operation of filter apparatus 50 during the service cycle, onlyvalves 55 and 57 are opened, all others are closed. In this manner,untreated liquid enters the filter vessel 52 through inlet line 54 tothe filter compartment 62. The pressure of the incoming untreated liquidforces it through the precoat layer, the filter elements 68, and theplenum compartment 64 into the outlet line 56 to a suitable service line(not shown). As the untreated water passes through the precoat layer andfilter elements, undissolved solids are removed. Dissolved solids arealso removed, if the precoat layer includes adsorbents and/orabsorbents. As the service cycle continues, the ability of the precoatand filter elements to remove contaminants diminishes, and theresistance to liquid flow increases due to the accumulation of solids onand within the precoat layer, and within the filter elements.

As well known in the art, the precoat material may contain variousreactive materials, such as ion exchange resins, activated carbon, etc.,or non-reactive materials, or combinations thereof.

Eventually the precoat becomes exhausted or the resistance to liquidflow becomes impractical. At such time, the filtering or service cycleis stopped. It is now necessary to clean the filter elements 68, byremoving the filter cake and the precoat layer therefrom, before thefilter elements 68 are precoated and the filter apparatus 50 returned toservice. The present invention is particularly directed to a method andapparatus to optimize the effectiveness of this cleaning cycle.

Referring again to FIG. 2, a preferred embodiment of the cleaning orbackwash method in accordance with the principles of the invention willnow be described. The upper end portions 80 of the filter elements 68extend through openings 72 in tube sheet 66 into plenum compartment 64.End portions 80 are spaced a short distance above the surface of tubesheet 66. A backwash inlet line 82, in communication with a source ofbackwash water (not shown), communicates with plenum compartment 64. Abackwash water inlet valve 84 controls the flow of backwash water intoplenum compartment 64.

A gas surge line 86 in communication with a pressurized gas surgechamber 88, communicates with plenum compartment 64. A gas surge valve90 controls the flow of pressurized gas into plenum compartment 64. Thepressurized gas is directed into chamber 88 through an inlet line 92controlled by valve 93.

A gas inlet line 94 is provided to selectively supply a pressurized gas,from a source of pressurized gas (not shown) into filter compartment 62.A regulator valve 96 is provided in line 94 to selectively control thegas pressure in filter compartment 62 for reasons that will hereinbelowbecome more apparent. A vent line 98, controlled by a vent valve 100,communicates with an upper portion of filter compartment 62 to vent gasfrom compartment 62 upon opening valve 100 to reduce the gas pressuretherein.

A bubble cap member 102 is preferably located above the upper end 80 ofthe filter element 68 in covering relationship thereto. The lower edgesof cap member 102 is spaced a short distance above the upper surface oftube sheet 66.

The discussion of the operation of the backwash method and apparatus ofthe present invention that hereinbelow follows describes the operationof the system and discloses certain novel relationships and operatingconditions which form a part of the invention.

At the initiation of the cleaning o backwash cycle, the filtercompartment 62, the plenum compartment 64 and the vessel 52 are fullwith liquid. The backwash cycle is initiated by opening vent valve 75and drain valve 77 to partially or preferably fully drain the liquidfrom the filter compartment 62 of vessel 52. During the drain step,backwash liquid may be periodically or continuously introduced at a lowflow rate through line 82 into plenum compartment 64 and then throughthe filter elements 68 in the reverse direction to normal flow to removethe spent resin and crude from the filter elements.

Upon completion of the drain step, valves 75 and 77 are closed andregulator valve 96 is open to direct pressurized gas into filtercompartment 62. Regulator valve 96 is adjusted to achieve a pressure incompartment 62 sufficient to support a predetermined volume of backwashliquid in plenum compartment 64 having a liquid level indicated as "X"in FIG. 2. The liquid level may be increased or decreased by increasingor decreasing the pressure in compartment 62. In so doing, the volume ofbackwash liquid utilized during each air surge cycle may be selected.Valves 84 and 75 are open and backwash liquid is directed into theplenum compartment 64 through line 82 until the predetermined volume ofbackwash liquid is established above tube sheet 66.

At this point in the backwash cycle, the filter elements 68 and thefilter compartment 62 are substantially free of liquid. Valve 84 is thenclosed and valves 90, 100 and 77 are quickly opened to increase thepressure in compartment 64 and decrease the pressure in compartment 62creating a pressure differential therebetween. In so doing a drivingforce is achieved for movement of the volume of backwash liquid. Sincecompartment 64 and the filter elements 68 are free of liquid, anunimpeded piston like flow of the backwash liquid from above the tubesheet 66 surges down the core of each of the filter elements. As therapidly moving piston of liquid reaches the bottom of each filterelement, the kinetic energy of the liquid is converted to a highpressure wave which reflects up the column of liquid in each filterelement. As the wave moves upwards, instantaneous high radial flowsurges are created at the filter surface of each filter element.

Following the kinetic air surge step, backwash liquid flow is sustained,unimpeded by liquid in the filter chamber.

The aforementioned kinetic air surge step may be repeated a sufficientnumber of times to thoroughly clean the filter elements 68. Duringsubsequent kinetic surge steps, it may be desirable to allow the filtercompartment 62 to partially fill with backwash liquid immediately priorto initiation of the next backwash step. By so doing, turbulence iscreated within compartment 62 which helps to break up large chunks ofmaterials and facilitates the removal thereof from vessel 60.

After the filter elements 68 have been thoroughly cleaned, the filtercompartment 62 is drained for the last time and the hereinabovedescribed precoating procedure is initiated.

In accordance with a presently preferred embodiment, the fixed volume ofbackwash liquid is in the range of 1.0 to 3.0 times the total core voidvolume of the filter elements.

The above described method and apparatus in accordance with the presentinvention solves a long known problem of in-situ cleaning of the filterelements of top tube sheet filters. The method and apparatus of theinvention provides a means of effectively directing the energy to thebackwash liquid to create a driving force for the movement of thebackwash liquid in a rapidly moving piston-like flow of liquid surgedownwardly through the core of the filter elements and radially outwardthrough the filter surfaces thereof. By utilizing air pressure toestablish the desired volume of backwash liquid, a different volume maybe achieved by merely adjusting the air pressure. Such utilization ofair pressure also minimizes the size of the bubble cap 102 and reducesthe cost thereof.

Many modifications and variations of the present invention ashereinabove set forth will occur to those skilled in the art, and it isintended to cover in the appended claims all such modifications andvariations as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A method for cleaning a plurality of filterelements having inner cores vertically positioned within a filter vesselwhich has a tube sheet dividing the filter vessel into a lower filtercompartment and an upper plenum compartment, said filter elements beingpositioned within said filter compartment and having upper portions thatare in fluid communication with said plenum compartment through openingsformed in said tube sheet, said method comprising:draining substantiallyall of the liquid from the filter compartment; establishing apressurized condition in said filter compartment and said filterelements that is sufficient to support a predetermined volume ofbackwash liquid in said plenum compartment above said tube sheet;increasing the pressure in said plenum compartment and reducing thepressure in said filter compartment so as to cause said predeterminedvolume of backwash liquid to move rapidly downward through the cores ofsaid filter elements and surge radially outward through the filtersurfaces thereof.
 2. The method as defined in claim 1 wherein said fixedvolume of backwash liquid is in the range of 1.0 to 3.0 times the totalcore void volume of said filter elements.
 3. The method as defined inclaim 2 wherein said preceding steps are repeated a plurality of times.4. Apparatus for filtering liquids, comprising:a filter vessel, a tubesheet dividing said vessel into a lower filter compartment and an upperplenum compartment, a plurality of filter elements vertically mounted insaid filter compartment, said filter elements having upper portions thatcommunicate with said plenum compartment through openings formed in saidtue sheet, said upper portions of said filter elements extend a shortdistance into said plenum compartment and a cap member is supported incovering relationship and spaced from said upper portions of said filterelements, said cap members having lower end portions spaced from saidtube sheet; means for draining at least a portion of the liquid from thefilter compartment; means for establishing a pressurized condition insaid filter compartment and said filter elements that is sufficient tosupport a predetermined volume o backwash liquid in said plenumcompartment above said tube sheet; means for directing saidpredetermined volume of backwash liquid into said plenum compartment;means for increasing the pressure in said id plenum compartment andmeans for reducing the pressure in said filter compartment so as tocause said fixed volume of backwash liquid to move rapidly downwardthrough the cores of said filer elements and surge radially outwardthrough the filter surfaces thereof.
 5. The invention as defined inclaim 4 wherein said means for establishing a pressurized condition insaid filer compartment and said filer elements includes an air supplyconduit having a first end in fluid communication with an air source anda second end in fluid communication with said filer compartment.
 6. Theinvention as defined in claim 5 wherein said air supply conduit includesa regulator means to selectively control the air pressure in said filtercompartment.
 7. The invention as defined in claim 4 wherein said meansfor increasing the pressure in said plenum compartment includes anexternal air surge chamber containing pressurized air that is in fluidcommunication with said plenum compartment through an air surge conduit.8. The invention as defined in claim 7 wherein said air surge conduitincludes an air surge valve for controlling the air pressure in saidplenum compartment.
 9. The invention as defined in claim 4 wherein saidmeans for reducing the pressure in said filter compartment includes avent conduit in fluid communication with said filter compartment havinga vent valve associated therewith.